Two insects circling horse that's monstrous (8)
I believe the answer is:
behemoth
'that's monstrous' is the definition.
(legendary enormous creature)
'two insects circling horse' is the wordplay.
'two insects' means two replacements for 'insect'.
'insect' becomes 'bee' (bee is a kind of insect).
'insect' becomes 'moth' (moth is a kind of insect).
'circling' means one lot of letters goes inside another (circling can mean going around or containing).
'horse' becomes 'h' (synonyms).
'bee'+'moth'='beemoth'
'beemoth' going around 'h' is 'BEHEMOTH'.
(Other definitions for behemoth that I've seen before include "Huge or monstrous creature" , "Huge animal as from the Bible" , "Something abnormally large and powerful" , "Abnormally large creature" , "Huge or monstrous person or thing" .)